Stringent limits on the magnetic field strength in the disc of TW Hya. ALMA observations of CN polarisation

Despite their importance in the star-formation process, measurements of magnetic field strength in proto-planetary disks remain rare. Linear polarisation of dust and molecular lines can give insights into the magnetic field structure, but only observations of the circular polarisation produced by Zeeman splitting can provide a direct measurement of magnetic field strenghts. This paper presents the first Atacama Large Millimeter/submillimeter Array (ALMA) observations of the Zeeman splitting of CN in the disk of TW Hya, the closest pre-main sequence star with a proto-planetary disk. The observations indicate an excellent polarization performance of ALMA, but fail to detect significant polarization. The analysis of the non-detection indicates a vertical component of the magnetic field of less than 0.8 mG, and a toroidal component of less than 30 mG. These limits rule out some of the earlier accretion disk models, but remain consistent with most recent detailed models that feature effcient advection.